In recent years, hybrid or electric automobiles have become widespread in terms of energy saving and construction machinery has also been increasingly hybridized. In general, construction machinery, such as hydraulic excavators, driven by a hydraulic system is equipped with a hydraulic pump enabling maximum-load work and a large-sized engine driving this hydraulic pump so that the machinery can cope with every work from light-load work to heavy-load work.
However, with respect to construction machinery, heavy-load work, such as heavy excavation work, in which earth and sand is frequently digged and loaded is part of the entire work. During light-load work, such as horizontal tow operation, for smoothing the ground, the capability of an engine is redundant. This is one of the factors making it difficult to reduce a fuel consumption of hydraulic excavators. In consideration of the foregoing, there is known hybrid construction machinery in which an engine is reduced in size to reduce a fuel consumption and the engine is assisted by outputs of a power storage apparatus and an electric motor to make up insufficient output due to the engine size reduction. The electric equipment, such as the power storage apparatus and the electric motor, constituting this hybrid construction machinery requires appropriate temperature control for thermal protection for a drive circuit and highly efficient operation.
In particular, the power storage apparatus has an upper-limit temperature under which the apparatus can be used without a current limit but the output of the power storage apparatus is reduced at a low temperature. To use a power storage apparatus without incurring output reduction in the power storage apparatus, it is necessary to warm the power storage apparatus to a predetermined temperature or higher. For example, Japanese Patent Application Laid-Open No. 2008-290636 (Patent Literature 1) describes a hybrid car including: a water-cooled engine and a motor driving the car; a battery pack (power storage apparatus) supplying electric power to the motor; an engine radiator coupled to a cooling water passage of the water-cooled engine and circulating refrigerant liquid between the radiator and the water-cooled engine; and a heat exchanger coupled to a cooling water passage of the water-cooled engine through a bypass valve and warming up the power storage apparatus with the refrigerant liquid circulated to the water-cooled engine. In the hybrid car, waste heat of the engine is utilized to warm up the power storage apparatus.